Current carrying coils are utilized in a variety of applications, including linear and other motors, generators, electromagnets, transformers and the like. While devices employing such coils (hereinafter sometime referred to as "coil devices") find extensive use, a limitation on the performance of such devices, and therefore on even more wide-spread use thereof, is thermal limitations resulting from the heat generated when a current flows through an electric coil. This heat must be dissipated effectively so that the coil device does not get excessively hot, such excessive heat resulting in potential destruction of the device. Coil devices are therefore generally rated for the highest continuous current they can withstand while maintaining an acceptable temperature level. This maximum current limits the performance of the device, for example limiting the maximum force obtained from a given motor or the maximum power a transformer can safely handle. Where more force or power is required, either a bigger device with more coils must be used or multiple devices must be employed. In either event, the resulting device will be bulkier, taking up more space in areas where space is frequently at a premium, heavier, which may be a problem in mobile applications, and generally significantly more expensive.
Improved thermal efficiency for devices utilizing current carrying coils thus facilitates an increased output per unit volume of the device. Another advantage of a more thermally efficient coil device is that, for the same current and output, the device will run cooler; and, since the coil resistance increases with temperature, increasing for example approximately 40% for a 100.degree. C. temperature rise, more efficient operation of the coil is obtained at lower temperatures. Insulation materials also degrade faster at high temperature, a rule of thumb being that every 10.degree. C. rise in temperature halves the insulation lifetime. Thus, improved thermal efficiency for a motor, transformer or other current carrying coil device allows for increased power efficiency, increased output/maximum motor force for a given size device, a smaller device for a given output, increased device life, increased machine accuracy (less distortion from thermal expansion) and/or some combination of the above.
Many efforts have heretofore been made to provide cooling for such coil devices. The simplest, and most common, is to let surrounding air freely convect away the heat. Fins may be added to improve this free convection by increasing the surface area exposed to air and further improvement can be achieved by forced convection of air over the coils or fins. Heat has also been conducted away from the coils by heat sinking a thermally conductive core in contact with the windings, by placing the coil in contact with a stationary or flowing fluid, such as air, an inert gas, water or oil, or by other similar techniques.
However, in all of the above schemes, the cooling medium, whether air, some other fluid, or a physical heat sink, only directly contacts the outer-most or inner-most layer of wires in the coil; thus, the heat generated by the many interior layers of the coil can not easily escape, this heat having to travel across multiple layers of the coil before reaching the cooling medium. The thermal resistance across coil layers is often substantial, leading to unacceptably high temperature gradients within the coil. This is at least in part because the air voids which typically exist between layers in a coil are good thermal insulators. While coils are often potted in an epoxy resin to eliminate the air voids, and thus improve heat transfer, such potting also increasing coil strength and the electrical insulation between wires, the thermal resistance of the epoxy, while lower than that for air, is still significant, and this thermal resistance, coupled with that of the insulation layers of the wires themselves, still results in a relatively large thermal resistance between coil layers. Since even a single hot spot in a coil can destroy insulation at that point, and thus destroy the coil, (and frequently the entire coil device), such existing cooling schemes, while an improvement over passive air convection cooling, still leave much to be desired in optimizing the thermal efficiency, and thus the performance, of coil devices.
To overcome the above problems, some large transformers or other large coil devices have used hollow wires through which a cooling medium is flowed. However, this is an expensive cooling technique and has generally only been practical for large machines. Other large machines have provided selective spacing between coil wires, through which spacings a cooling medium may be flowed or into which spacing some heat sink medium may be placed. While such an arrangement can be used in certain large transformer applications, motors and most other coil devices operate far more efficiently when the coils are together rather than when they are separated, so that this approach has not proved practical, and has not generally been utilized, for smaller coil devices, such as those involving linear motors where such loss of efficiency is unacceptable.
An improved method and apparatus for cooling the coils of coil devices in general, and such smaller devices in particular, to improve the thermal efficiency thereof without significant negative impact on the coil device's efficiency is therefore required.